Numerous instances exist in which the heating or cooling of a volume is desired. For example, the heating of a volume of space can provide environmental comfort, or the cooling of a spatial volume may provide protection against the overheating of electronic circuitry. In the past such devices have relied upon electrical power to drive mechanical compressors that enable one or more thermodynamic operational cycles to be sustained. Since moving mechanical parts can impair reliability and shorten device life, while the availability of electrical power is limited in certain applications such as those in space, heat pumps or coolers with no moving parts and in which the need for electrical power is reduced or even entirely eliminated would be highly desirable.
An achievement which has enabled advances to be made in the field of thermal engines was the development of the osmotic heat pipe. In an osmotic heat pipe two compartments, one containing a solution and the other containing pure solvent, are separated by a membrane which is permeable to the solvent only. The solution compartment is also connected to the solvent compartment through a path including an evaporator and a condensor. When heat is applied to the solution, solvent evaporates creating a concentration gradient which induces migration of the solvent near the membrane to the region of evaporation. The solute concentration becomes greater near the membrane, and fluid motion of solvent through the membrane is induced. The solvent vapor flows to the condensor where it condenses, giving up heat, after which it returns to the solvent compartment and flows through the membrane into the solution compartment. A steady-state condition prevails when the solvent evaporation rate is balanced by the solvent flow rate through the membrane.
An early osmotic heat pipe is disclosed in U.S. Pat. No. 3,677,337 to L. L. Midolo, and a detailed description of the theory, operation, and construction of osmotic heat pipes is given in the paper by C. P. Minning et al. "Development of an Osmotic Heat Pipe", AIAA Paper No. 78-442, 3rd International Heat Pipe Conference, Palo Alto, Calif., May 22-24, 1978. A further discussion of osmotic heat pipes including their application for heating and cooling is given in the paper by A. Basiulis et al., "Osmotically Pumped Energy Transport System", AIAA Paper No. 80-0210, AIAA 18th Aerospace Sciences Meeting, Pasadena, Calif., Jan. 14-16, 1980. Additional developments in osmotic heat pipe technology are described in the paper by A. Basiulis et al., "Design, Development, and Test of a 1000 Watt Osmotic Heat Pipe", AIAA Paper No. 80-1482, AIAA 15th Thermophysics Conference, Snowmass, Colo., Jul. 14-16, 1980, and adaptation of osmotic heat pipe technology to space applications is given in the paper by H. J. Tanzer et al., "Osmotic Pumped Heat Pipes for Large Space Platforms", AIAA Paper No. 82-0902, AIAA/ASME 3rd Joint Thermophysics, Fluids, Plasma and Heat Transfer Conference, St. Louis, Mo., June 7-11, 1982.
Improved osmotic heat pipes have been developed wherein passage of a convection current across the membrane is induced on the solution side by displacing freshly pumped solvent with a rich solute-solvent mixture. One arrangement for inducing such a convection current, disclosed in U.S. Pat. No. 4,331,200 to A. Basiulis, utilizes a pair of concentric tubes extending from the membrane region to the evaporator for establishing separate paths by which a weak solution diluted with freshly pumped solvent is caused to move from the membrane to the evaporator and a rich solute-solvent mixture is caused to move from the evaporator to the membrane. Another arrangement for achieving such a convection current is disclosed in U.S. Pat. No. 4,365,664 to A. Basiulis and provides separate lean and rich solution paths between the evaporator and the solution compartment, the rich solution path having a higher pressure drop than the lean solution path.